This invention relates to a method and apparatus for cooling hot steel pipe without causing the pipe to bend along its length so as to damage the roundness of its cross section.
When steel pipe is cooled rapidly from such a high temperature as, for example, 850.degree. C. for the purpose of heat treatment, the pipe may deform unless the cooling proceeds evenly in the circumferential and axial directions thereof.
Deformation of steel pipe occurring during the cooling process can be classified as "bend" which is the impairment of straightness in the axial direction and "elliptical deformation" which is the impairment of roundess in the cross-sectional plane.
The bent or elliptically deformed pipe handling in the subsequent process is difficult or impossible to handle during subsequent processing.
The two kinds of deformation developed during the heat treatment process can be corrected by the described in the following. application of corrective mechanical force on a cold pipe. This however, leaves internal stress within the pipe.
When used in deep oil wells, wells producing high-pressure gases, sour oils and gas wells, and wells in cold districts and in other hostile conditions, pipes not free of internal stress may collapse under low pressure or develop stress-corrosion cracking. Therefore, cold-correction is not always desirable depending upon the kind of service into which pipes are put.
The pipe deformation developed during the cooling process can be corrected to a considerable extent; by, for correcting the bend, by straightening, and for correcting the elliptical deformation, by warm sizing immediately after tempering. Yet, a certain amount of detrimental deformation remains unremoved sometimes. If a thread is cut at the end of such a pipe after heat treatment, the thread will not turn out to be satisfactory.
The bend of pipe is commonly corrected by use of a multi-roll straightener comprising concave-drum-shaped rolls set in an intersecting fashion. The multi-roll straightener can straighten a long-order bend extending throughout the entire length of a pipe with high accuracy. However, this method is capable of improving any minor bend at the pipe end only approximately 50 percent because of the limitations imposed by its roll arrangement.
Turning that is given to pipes being conveyed or waiting in the walking-beam type tempering furnace followed the quenching process also corrects a long-order bend along the pipe length to some extent, but this method also is not very effective for a minor pipe-end bend.
When such a pipe-end bend is left uncorrected even after tempering or straightening, no good straightness or satisfactory thread cutting can be hoped for in the finished pipe. This pipe-end bend shows a strong tendency to appear in small-diameter, light-wall pipes, such as those having an outside diameter not larger than 100 mm.
Elliptical deformation of a pipe is usually corrected by passing it, after tempering, through a sizing mill while applying a small amount of reduction, which commonly comprises three stands each of which has two or three rolls forming a circular pass.
But any pipe the cross-section of which became heavily elliptical in the quenching process passes through this mill uncorrected to the subsequent process.
This multi-roll straightener also corrects the roundness of a pipe when it straightens its bend, but only to the extent of approximately 50 percent.
Like the axial bend, the elliptical deformation also has an adverse effect on the thread cutting at pipe ends and the collapse strength of pipe in high pressure wells.
Elliptical deformation occurs mainly on larger-diameter pipes.
For the reason described previously, high-grade seamless steel pipes for oil-well applications tolerate very little deformation. Therefore, they call for a cooling means which cause little or no deformation in the cooled pipe.
This invention aims at providing a cooling means which ensures the production of steel pipes having little or no deformation.